Na3MnZr(PO4)3: A High-Voltage Cathode for Sodium BatteriesHongcai Gao,†,# Ieuan D. Seymour,‡,# Sen Xin,† Leigang Xue,† Graeme Henkelman,‡

and John B. Goodenough*,†

†Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States‡Department of Chemistry and the Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin,Texas 78712, United States

*S Supporting Information

ABSTRACT: Sodium batteries have been regarded aspromising candidates for large-scale energy storage applica-tion, provided cathode hosts with high energy density andlong cycle life can be found. Herein, we report NASICON-structured Na3MnZr(PO4)3 as a cathode for sodium batteriesthat exhibits an electrochemical performance superior to thoseof other manganese phosphate cathodes reported in theliterature. Both the Mn4+/Mn3+ and Mn3+/Mn2+ redoxcouples are reversibly accessed in Na3MnZr(PO4)3, providinghigh discharge voltage plateaus at 4.0 and 3.5 V, respectively. A high discharge capacity of 105 mAh g−1 was obtained fromNa3MnZr(PO4)3 with a small variation of lattice parameters and a small volume change on extraction of two Na+ ions performula unit. Moreover, Na3MnZr(PO4)3 exhibits an excellent cycling stability, retaining 91% of the initial capacity after 500charge/discharge cycles at 0.5 C rate. On the basis of structural analysis and density functional theory calculations, we haveproposed a detailed desodiation pathway from Na3MnZr(PO4)3 where Mn and Zr are disordered within the structure. Wefurther show that the cooperative Jahn−Teller distortion of Mn3+ is suppressed in the cathode and that Na2MnZr(PO4)3 is astable phase.

■ INTRODUCTION

The integration of electric power generated from renewablesunlight and wind energy cannot be used unless it is stored inlarge-scale rechargeable batteries to smooth the supply of thiselectric power into the grid.1,2 The ubiquitous availability ofsodium from the oceans makes sodium batteries preferable tolithium batteries provided cathode hosts for Na+ extraction/insertion can be identified with sufficiently high voltage versusNa+/Na, a long cycle life, and fast charge/discharge rates.3−5

The stored energy density is <V(q)> × Q(Idis) at a constantcurrent Idis = dq/dt, where <V(q)> is the cell voltage averagedover the state of charge q and the capacity Q(Idis) is thenumber of electrons transferred from the anode to the cathodeper unit weight and/or volume in the time Δt for theelectrochemical reaction at a desired rate between the twoelectrodes.6 Because the electrodes change volume during acharge/discharge cycle, a small volume change of the cathodesduring a charge/discharge cycle is preferred even with a liquidelectrolyte.A number of layered transition-metal oxides have been

investigated as cathodes for a sodium battery, but they havebeen plagued by large volume changes and phase instabilitiesduring cycling.7−9 However, transition-metal cyanides with aPrussian-blue structure provide low-cost, facile synthesis andlow activation energies for Na+ insertion/extraction.10−14

Another important category of Na+ cathode hosts consists ofpolyanionic transition-metal compounds with open-framework

structures.15−19 The strong binding in the polyanions can offerstructural stability and a relatively small volume change duringNa+ insertion/extraction over a large solid-solution range.20,21

Recently, several manganese-based polyanionic compoundshave been shown to exhibit an interestingly high voltage versussodium,22,23 but an unacceptable performance has beenattributed to Jahn−Teller distortions of Mn3+ and/or adisproportionation reaction of surface Mn, 2Mn3+ = Mn2+ +Mn4+, with Mn2+ dissolution into a liquid electrolyte.24

Moreover, most of the polyanion hosts have used only theMn3+/Mn2+ couples; the limited number operating on theMn4+/Mn3+ couple have provided little interest. The Mn4+/Mn3+ couple in Li2MnP2O7 is located at a voltage that oxidizesconventional organic-liquid electrolytes.25 The Mn4+/Mn3+

couple is only partially (30%) active in Na3MnPO4CO3 andcannot be accessed in Na4MnV(PO4)3 from which theextraction of Na+ ions proceeds in the sequences of the V4+/V3+, Mn3+/Mn2+, and V5+/V4+ redox couples from the low tothe high voltage.26,27 Very little cycle stability was obtained inNa3MnTi(PO4)3 and Na3Co0.5Mn0.5Ti(PO4)3.

28,29

In order to reduce the concentration of Mn in a polyanionframework structure to where the cooperative Jahn−Tellerdistortion does not occur with all Mn3+ and the surfacedisproportionation of the Mn3+ to Mn2+ and Mn4+ might be

Received: October 22, 2018Published: December 2, 2018

Article

pubs.acs.org/JACSCite This: J. Am. Chem. Soc. 2018, 140, 18192−18199

© 2018 American Chemical Society 18192 DOI: 10.1021/jacs.8b11388J. Am. Chem. Soc. 2018, 140, 18192−18199

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suppressed, we have investigated as a sodium cathodeNa3MnZr(PO4)3 having the rhombohedral NASICON struc-ture. In an early report, the measurement of ionic conductivityof Na3MnZr(PO4)3 revealed that the rhombohedral phase ofNa3MnZr(PO4)3 has an activation energy of 0.38 eV, which issimilar to the activation energies of rhombohedral phases ofNa3V2(PO4)3 (0.34 eV) and Na3Cr2(PO4)3 (0.39 eV) andconsiderably smaller than the activation energies of themonoclinic phases of Na3V2(PO4)3 (0.87 eV) andNa3Cr2(PO4)3 (0.62 eV), suggesting its potential suitabilityas a cathode for sodium batteries.30−33 However, in this earlystudy, Na3MnZr(PO4)3 was not investigated electrochemicallyas a cathode material of a sodium battery.30 A subsequentstudy failed to obtain a reversible extraction of Na+ fromNa3MnZr(PO4)3 synthesized by solid-state reaction anduncoated by carbon or any other electronic conductor, whichhas led to the suggestion that the problem is an instability ofthe octahedral-site Mn3+ ion.33 Such an instability could be dueto a cooperative Jahn−Teller distortion or to a surfacedisproportionation 2Mn3+ = Mn2+ + Mn4+ with Mn2+

dissolution in the liquid electrolytes. The failure to extractNa+ reversibly was the result of a solid-state synthesis that givesmicron-sized crystallites and a failure to coat the activeelectrode material that is an electronic insulator with a carboncoat, or a coat of another electronic conductor that also allowsNa+ cross over.34−36 In this paper, we have used a sol−gelsynthesis to obtain 200 nm particles of Na3MnZr(PO4)3 thatwere coated in situ with a thin carbon layer. With theseparticles, we were able to extract/insert Na+ ions reversiblyover a large solid-solution range, accessing not only the Mn3+/Mn2+ redox couple but also the Mn4+/Mn3+ redox couple at,respectively, 3.5 and 4.0 V versus Na+/Na. Moreover, thecathode showed no cooperative Jahn−Teller distortion downto lowest temperatures and it can be cycled stably over manycycles indicating no evidence of surface disproportionation andMn2+ dissolution. Moreover, we report a theoretical analysis ofthe Na+-ion extraction sites in the rhombohedral NASICON-structured cathode Na3MnZr(PO4)3 containing two quitedifferent octahedral-site transition-metal cations whereasearlier theoretical studies have been confined to NASICONstructures with only one octahedral-site transition-metal cation.The first-principles calculations also demonstrate the intrinsicstability of the Na3MnZr(PO4)3 structure during desodiationin addition to the changes in the local Mn environment.

■ RESULTS AND DISCUSSIONSynthesis and Crystal Structure of Na3MnZr(PO4)3. A

sol−gel method was used to prepare the cathode materialNa3MnZr(PO4)3; the resulting material had a particle size ofabout 200 nm (Figure 1a). A thin layer of carbon was coatedon the surface of Na3MnZr(PO4)3 particles during thesynthesis procedure to provide electron tunneling to thesurface Na+ ions with subsequent oxidation of the Mn2+ toMn3+ and, eventually, to Mn4+ (Figure 1b); the amount ofcoated carbon was about 7.1% according to thermogravimetricanalysis (Figure S1). The crystalline region of the cathodematerial exhibited lattice fringes with an interplanar spacing ofabout 0.64 nm, consistent with the (012) atomic planes ofNa3MnZr(PO4)3 with the rhombohedral structure (Figure 1c).A Rietveld refinement of the X-ray diffraction (XRD)

pattern further confirmed the formation of Na3MnZr(PO4)3with the rhombohedral NASICON structure in the spacegroup R3̅c (Figure 1d).30,37 The absence of additional peaks

corresponding to long-range ordering between Mn and Zr inthe XRD pattern indicates the octahedral sites were randomlyoccupied by the two kinds of transition metals in theNASICON framework.38,39 The lattice parameters ofNa3MnZr(PO4)3 (a = 8.988 Å, and c = 22.594 Å) are largerthan those of the well-documented Na3V2(PO4)3 cathode forsodium batteries (a = 8.738 Å, and c = 21.815 Å),40 whichprovides sufficient interstitial volume for extraction/insertionof Na+ ions. The Rietveld refinement revealed the occupancyfactors of 0.558, 0.596, and 0.066 for Na(1), Na(2), and Na(3)sites, respectively, corresponding to 16.45 (6 × 0.558 + 18 ×0.596 + 36 × 0.066) Na+ ions per unit-cell (Z = 6), consistentwith the expected chemical formula Na3MnZr(PO4)3 (TableS1).The crystal structure of Na3MnZr(PO4)3 is composed of

MnO6 and ZrO6 octahedral units bridged by corner-sharingPO4 tetrahedral units to establish a framework structure withinterstitial sites accommodating three different types of Na+

ions: (i) the Na(1) sites with 6-fold coordination, (ii) theNa(2) sites with 8-fold coordination, and (iii) the Na(3) siteswhich are four-coordinate (NaO4) sites situated in-betweenthe Na(1) and Na(2) sites (Figure 1e). An occupancy ofNa(1) sites less than unity in the Rietveld refinementsuggested that a portion of Na+ ions at Na(1) sites aredisplaced to the Na(3) sites, which has been observed insimilar NASICON structures and suggests that simultaneousoccupation of neighboring Na(1) and Na(2) sites by Na+ ionsresults in a Coulombic repulsion that displaces a Na+ ion fromthe Na(1) site to a Na(3) site.41 The electrochemical activitiesof NASICON-structured materials are typically from extrac-tion/insertion of Na+ ions located at Na(2) sites.42,43 As isdetailed in the following sections, the energies of the Na(1)and Na(2) sites are affected by the local Mn and Zr ordering. Atheoretical capacity of 107 mAh g−1 could be obtained forNa3MnZr(PO4)3 through utilization of both the Mn3+/Mn2+

and Mn4+/Mn3+ redox couples with extraction/insertion of twoNa+ ions at Na(2) sites per formula unit.

Electrochemical Performance of Na3MnZr(PO4)3. Twooxidation peaks at 3.6 and 4.1 V are revealed in the cyclicvoltammetry profiles of the Na3MnZr(PO4)3 cathode,corresponding to the reversible extraction of two Na+ ionsaccompanied by the successive two-phase transformations

Figure 1. (a) SEM image, (b) TEM image, (c) high-resolution TEMimage, (d) XRD pattern and Rietveld refinement, and (e) crystalstructure of the cathode material Na3MnZr(PO4)3.

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from Na3MnZr(PO4)3 to Na2MnZr(PO4)3 and then toNaMnZr(PO4)3 (Figure 2a). The two reduction peaks located

at 4.0 and 3.5 V can be attributed to the successive Mn4+/Mn3+

and Mn3+/Mn2+ redox couples, accompanied by the trans-formations from NaMnZr(PO4)3 to Na2MnZr(PO4)3 and thenback to Na3MnZr(PO4)3. The charge profile of the cathodeexhibits a voltage plateau at 3.6 V corresponding to theextraction of the first Na+ ion through the Mn3+/Mn2+ redoxcouple followed by another voltage plateau at 4.1 Vcorresponding to the extraction of the second Na+ ion throughthe Mn4+/Mn3+ redox couple (Figure 2b). Upon discharge, thetwo plateaus located at 4.0 and 3.5 V represent the insertion oftwo Na+ ions through the Mn4+/Mn3+ and Mn3+/Mn2+ redoxcouples, respectively. As a cathode material for sodiumbatteries, Na3MnZr(PO4)3 delivers discharge voltage plateaushigher than the well-documented NASICON compoundsNa3V2(PO4)3 (3.4 V) and NaTi2(PO4)3 (2.1 V).44,45 More-over, the Mn4+/Mn3+ redox couple in Na3MnZr(PO4)3provides a discharge voltage (4.0 V) higher than the well-documented Mn3+/Mn2+ redox couple in other manganesephosphate cathodes, such as Na2MnP2O7 (3.6 V) andNa4Mn3(PO4)2P2O7 (3.8 V) for sodium batteries.46−48

The discharge capacity of Na3MnZr(PO4)3 was 105 mAhg−1 at 0.1 C rate, which is close to the theoretical capacity (107mAh g−1) corresponding to extraction/insertion of two Na+

ions per formula unit (Figure 2c). When the rate increased to 1and 10 C, the discharge capacity of the cathode was 82 and 52mAh g−1, respectively. The low Coulombic efficiency at thelow rate (∼90% at 0.1 C) is most probably from the oxidationof the liquid electrolyte at the high charging voltage (4.3V);49−51 however, Na3MnZr(PO4)3 still exhibits an initialCoulombic efficiency (87%), higher than that of othermanganese phosphate cathodes for sodium batteries, such asNa2MnP2O7 (69%), Na4Mn3(PO4)2P2O7 (60%), andNa2MnPO4F (80%).48,52,53 When the current density wentback to the initial 0.1 C rate, the discharge capacity of thecathode returned to the value of 104 mAh g−1, demonstrating auseful rate capability of the cathode. In addition, an excellentcycling stability was obtained for Na3MnZr(PO4)3; 91% of the

initial capacity could be retained after 500 cycles at 0.5 C ratewith an average Coulombic efficiency of 98.7% (Figure 2d). Interms of discharge voltage, reversible capacity, and cyclingstability, Na3MnZr(PO4)3 gives a performance superior to thatof other manganese phosphate cathodes for sodium batteriesreported in the literature (Table S2). The XRD pattern andTEM image of the cathode after the charge/discharge cyclingtests were almost identical to those of the pristine one (FigureS2), which implies that the crystal structure of Na3MnZr-(PO4)3 was well-maintained after repeated extraction/insertionof Na+ ions.

Desodiation Mechanism of Na3MnZr(PO4)3. In order tounderstand further changes in the structure during desodiation,the energies of NaxMnZr(PO4)3 structures from x = 1−3 werecomputed with DFT. The convex hull shows the formationenergy per formula unit (Eform) of NaxMnZr(PO4)3 phasesrelative to the end member compositions of Na3MnZr(PO4)3(ENa3) and NaMnZr(PO4)3 (ENa1) according to the formulaEform = ENax − 1/2(x − 1) ENa3 − 1/2(3 − x) ENa1 (Figure 3a).

Na2MnZr(PO4)3 is a stable phase on the hull between the endmember configurations, and the intermediate configurations atNa1.5MnZr(PO4)3 and Na2.5MnZr(PO4)3 are metastable/unstable. Two-phase reactions between Na3MnZr(PO4)3 andNa2MnZr(PO4)3 and between Na2MnZr(PO4)3 and NaMnZr-(PO4)3 are therefore preferred, in accordance with XRDanalysis in the following section. From the lowest energystructures, sequential reactions from Na3MnZr(PO4)3 →Na2MnZr(PO4)3 and Na2MnZr(PO4)3 → NaMnZr(PO4)3were predicted to occur at 3.52 and 4.02 V based on theapproach of Aydinol et al.,54 in good agreement with the

Figure 2. Electrochemical performance of the Na3MnZr(PO4)3cathode. (a) Cyclic voltammetry profiles at a scan rate of 1 mV s−1.(b) Charge/discharge profiles at rates from 0.1 to 10 C. (c) Dischargecapacities and Coulombic efficiencies at rates from 0.1 to 10 C. (d)Charge/discharge cycling performance at 0.5 C rate.

Figure 3. (a) Convex hull of NaxMnZr(PO4)3 from DFT calculations.(b) Near hull DFT relaxed structure of Na3MnZr(PO4)3 containing aNa+ ion in a Na(3) site. (c) Structures of the lowest energy phase ofNaxMnZr(PO4)3 and the relationship between the unit cellparameters (a, b, c) of the DFT cells and the corresponding unitcell parameters of the R3̅c structure (aR, bR, cR). Na(1) vacancies areshown as dashed circles. (d) DFT optimized structures of Na2MnZr-(PO4)3 with different Mn/Zr orderings.

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experimentally observed charge plateaus at 3.6 and 4.1 V. Theenergy of the NaxMnZr(PO4)3 structures on the hull (x = 1, 2,and 3) were compared to all phases in the Na−Mn−Zr−P−Ophase diagram in the Materials Project,55 and all of thestructures were found to be stable with respect todecomposition to neighboring phases of NaMnPO4,NaZr2(PO4)3, Na4P2O7, NaPO3, MnPO4, and ZrO2, high-lighting the stability of the desodiated NaxMnZr(PO4)3structure.For the end member composition of Na3MnZr(PO4)3, the

small energy difference between the structures above the hullpredicated with DFT calculations (Ehull = 1−35 meV/atom)suggests that there is significant disorder between the Mn andZr sites within the structure. At the local scale, the distributionof Mn and Zr sites in Na3MnZr(PO4)3 leads to three possibletypes of Na(1) site that share faces with either two MnO6, twoZrO6 or one MnO6 and one ZrO6 octahedron along the c-axis.In the lowest energy structure of Na3MnZr(PO4)3, the MnO6−Na(1)−MnO6 and MnO6−Na(1)−ZrO6 sites are occupied,whereas the ZrO6−Na(1)−ZrO6 sites are vacant (Figure 3c).This result suggests that the unfavorable electrostaticinteraction between Zr4+ and Na+ may locally destabilizeZrO6−Na(1)−ZrO6 configurations in Na3MnZr(PO4)3. Thisfinding is consistent with the structural refinement in which theoccupancy of the Na(1) sites was less than unity. Duringstructural optimization of several low energy structures, Na+

ions initially located in a ZrO6−Na(1)−ZrO6 configurationsrelaxed to the intermediate four coordinated (NaO4) sitessituated between the Na(1) and Na(2) sites (Figure 3b), inaccordance with the Rietveld refinement. The formationenergy of the structure was only 4 meV/atom above thelowest energy structure, which suggests that these Na(3) sitesmay act as additional low-energy positions to accommodateNa+ ions in the Na3MnZr(PO4)3 structure.In the Na3MnZr(PO4)3 structures, the Na

+ ions occupy theNa(2) sites that are face sharing with either two Mn2+ or oneMn2+ and one Zr4+ octahedron, as a result of the smallerelectrostatic repulsion of Mn2+−Na+ interactions comparedwith Zr4+−Na+ interactions. The high energy, ZrO6−Na(2)−ZrO6 local configurations are therefore preferential sites forNa(2) vacancies. For a random distribution of Mn/Zr ions, theZrO6−Na(2)−ZrO6 configurations account for 1/4 of the totalNa(2) vacant sites. As approximately 1/3 of the total Na(2)sites are vacant in Na3MnZr(PO4)3, the remaining 1/12 ofNa(2) vacancies will preferentially occur in MnO6−Na(2)−ZrO6 configurations rather than in the MnO6−Na(2)−MnO6configurations, based on the previous electrostatic argumentsand analysis of the low energy structures. The energies of thestructures are also sensitive to the local Na ordering due toNa+−Na+ interactions. However, the random distribution ofMn/Zr ions and high energy of the ZrO6−Na(2)−ZrO6 sitesprecludes the formation of long-range Na orderings, such asthat observed in the rhombohedral structure ofNa3V2(PO4)3.

56

Analogous to Na3MnZr(PO4)3, there is a weak preferencefor Mn3+/Zr4+ ordering in Na2MnZr(PO4)3 (Figure 3d). Whilethe lowest energy structure of Na2MnZr(PO4)3 had fullyoccupied Na(1) sites, the Na(1) sites along ZrO6−Na(1)−ZrO6 or ZrO6−Na(1)−MnO6 configurations are also lessstable than MnO6−Na(1)−MnO6 configurations, and Na+

ions in ZrO6−Na(1)−ZrO6 or ZrO6−Na(1)−MnO6 config-urations in several structures relaxed to Na(3) sites during thestructural optimization, leading to vacant Na(1) sites along

either ZrO6−Na(1)−ZrO6 or ZrO6−Na(1)−MnO6 configu-rations. Na+ ions occupy 1/3 of the Na(2) sites inNa2MnZr(PO4)3, and the site containing a MnO6−Na(2)−MnO6 configuration will be preferentially occupied because ofthe lower energy than the site containing a Na(2) ion in aMnO6−Na(2)−ZrO6 or ZrO6−Na(2)−ZrO6 configuration.For the NaMnZr(PO4)3 end member, all of the Mn sites are

oxidized to Mn4+. In the lowest energy structure, the Na(1)sites are fully occupied whereas the Na(2) sites areunoccupied. In the small unit cell structures that initiallycontained vacancies in MnO6−Na(1)−MnO6 or ZrO6−Na(1)−ZrO6 configurations, adjacent Na+ ions in Na(2)sites relaxed to occupy the Na(1) sites. The lowest energystructure containing a ZrO6−Na(1)−MnO6 vacancy was 18meV/atom above the hull, suggesting that there is a strongpreference for full occupancy of the Na(1) sites in NaMnZr-(PO4)3.From these results, a tentative mechanism for the

desodiation of Na3MnZr(PO4)3 can be proposed. On initialdesodiation from Na3MnZr(PO4)3 to Na2MnZr(PO4)3 at 3.6V, Na+ ions are preferentially extracted from the partiallyoccupied MnO6−Na(2)−ZrO6 sites while MnO6−Na(2)−MnO6 sites remain fully occupied. The Na+ ions in localMnO6−Na(1)−MnO6 environments remain fully occupied inboth Na3MnZr(PO4)3 and Na2MnZr(PO4)3. The occupationof ZrO6−Na(1)−ZrO6 sites increases on desodiation toNa2MnZr(PO4)3, although displacement of Na(1) ions toNa(3) sites may also take place more easily due to the increasein the number of Na(2) vacancies. On further desodiation toNaMnZr(PO4)3 at 4.1 V, the Na(1) sites become fullyoccupied, while the Na(2) sites are fully unoccupied.

Structural Evolution of Na3MnZr(PO4)3. The phasetransition of Na3MnZr(PO4)3 during extraction/insertion ofNa+ ions was investigated by ex situ XRD analysis (Figure 4a).

A two-phase transformation from Na3MnZr(PO4)3 toNa2MnZr(PO4)3 was observed on extraction of the first Na+

ion, followed by another two-phase transformation processfrom Na2MnZr(PO4)3 to NaMnZr(PO4)3 on extraction of thesecond Na+ ion. The phase transition of Na3MnZr(PO4)3 isaccompanied by the changes in the unit cell parameters

Figure 4. (a) XRD patterns of the Na3MnZr(PO4)3 cathode atdifferent states of charge and discharge. (b) Variation of latticeparameters of NaxMnZr(PO4)3 structures. (c) Variation of volume ofNaxMnZr(PO4)3 structures.

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(Figure 4b), which are closely linked to the occupation of theNa(1) and Na(2) sites.57,58

There is a small overall increase in the c-lattice parameterduring desodiation (0.1%), in agreement with the DFTcalculation (0.4%) from the supercell containing 8 formulaunits of NaxMnZr(PO4)3 with disordered Mn and Zr sites(Figure S3). It is known from previous studies that thepresence of Na(1) vacancies leads to an expansion of the c-axis.59 Na3MnZr(PO4)3 contains Na(1) vacancies at theZrO6−Na(1)−ZrO6 sites. On desodiation to Na2MnZr(PO4)3,one of the Na+ ions remained in a ZrO6−Na(1)−ZrO6 site,while the other Na(1) site was empty as the Na+ ion occupiedan adjacent Na(3) site. On desodiation to NaMnZr(PO4)3, theNa(1) sites are filled and there is a small increase in the c-lattice parameter, which has previously been ascribed to anincreased repulsion of the transition metals (i.e., Mn4+−Mn4+

vs Mn2+−Mn2+) along the c-axis at higher states of charge. Thesmall overall change in the c-lattice parameter observedexperimentally during desodiation is related to the competingeffects of transition metal repulsion causing expansion andincreasing Na(1) site occupation causing contraction.The small increase in the c-lattice parameter during

desodiation is accompanied by a decrease in the a-latticeparameters and therefore an increase of c/a ratio. The decreasein the a-lattice parameter is commonly ascribed to a decreasein the Na(2) site occupation and a decrease in the effectiveionic radius of the transition metal during oxidation, which forthe current system are 0.83, 0.645, 0.53 Å for Mn2+, Mn3+, andMn4+, respectively.60 The overall decrease in the a-latticeparameter observed experimentally (2.8%) is smaller thanpredicted from the supercell (5.1%) and the lowest energystructures in the convex hull (5.9%), which may be due to theresidual transition-metal ordering in the calculated structuresin addition to the lack of temperature effects in the nominally 0K DFT calculations. Moreover, Na3MnZr(PO4)3 has a smallervolume change (5.5%) on desodiation to NaMnZr(PO4)3 thanother cathode materials, such as Na4Mn3(PO4)2P2O7 (∼7%)and Na3V2(PO4)3 (∼8.3%) for sodium batteries, and LiFePO4(∼6.8%) for lithium batteries (Figure 4c).52,61,62 In general,the small variation in the a and c lattice parameters and thesmall volume change of Na3MnZr(PO4)3 is key to the long-term cycling stability.The oxidation states of Mn and Zr in the Na3MnZr(PO4)3

electrode was probed by X-ray photoelectron spectroscopy(XPS) at different states of charge (Figure S4). The bindingenergy of Mn 3p located at 48.8 eV before charging is inagreement with the presence of Mn2+ in the pristineNa3MnZr(PO4)3 electrode. The binding energy of Mn 3pincreased to 49.5 eV after charging the electrode to 3.9 V,indicating the oxidation of Mn2+ to Mn3+. At the end ofcharging the electrode to 4.3 V, the binding energy of Mn 3pfurther increased to 50.2 eV, implying the oxidation of Mn3+ toMn4+.63 The binding energies of Zr 3d remained unchanged inthe charging process, which suggests that the extraction/insertion of Na+ ions proceeds through utilization of the Mn3+/Mn2+ and Mn4+/Mn3+ redox couples, keeping the valence stateof Zr4+ unchanged. The change in the oxidation state is furthersupported by calculation of the magnetic moment on the Mnand Zr sites of the lowest energy structures in the convex hullthrough a Bader analysis (Table S3).64 The magnetic momenton Zr remains zero through-out the desodiation process andthe charge remains approximately constant at +2.70 e,consistent with a fixed Zr4+ oxidation state with a d-electron

configuration of d0. The change in the magnetic moment onMn is consistent with the oxidation of high-spin Mn2+ (d5) toMn3+ (d4) from Na3MnZr(PO4)3 → Na2MnZr(PO4)3,followed by the oxidation of Mn3+ to Mn4+ (d3) fromNa2MnZr(PO4)3 → NaMnZr(PO4)3.

Inhibited Cooperative Jahn−Teller Distortion ofManganese. As all of the manganese ions in the Na2MnZr-(PO4)3 structure are in the Mn3+ oxidation state, it may beexpected that the octahedral MnO6 centers would undergo apositive (negative) Jahn−Teller distortion in which two (four)of the Mn−O bond are lengthened and four (two) of the Mn−O bonds are shortened, as is observed in other Mn3+

systems.65−67 There are two distinct O sites in therhombohedral NASICON structure: O1 and O2 (Figure 5a).

The O1 ions are located in between the Mn (or Zr) and Na(1)sites, forming the common faces between the MnO6 and NaO6octahedra. The O2 sites are on the opposite side of the MnO6(or ZrO6) octahedra along the c-direction of the rhombohedralcell.In the lowest energy Na2MnZr(PO4)3 structure, MnO6

centers have two different local environments (Mn1 andMn2). For the Mn1 configuration, four of the Mn1−O bondsare shortened (S) from the average bond length of 2.08 Å andtwo Mn1−O bonds are lengthened (L), which is consistentwith a positive Jahn−Teller distortion, however, there issignificant deviation between the Mn1−O1 and Mn1−O2bond lengths (Figure 5b). The Mn1−O1(L) bond, whichshares a common O1 with two Na(2) sites, is considerablylonger (2.37 Å) than the Mn1−O2(L) bond (2.07 Å) on theopposite site of the MnO6 octahedron that shares a commonO1 with one Na(2). The Mn1−O1(S) bonds in the planeperpendicular to the Mn1−O(L) bond are also slightly shorterthan the Mn1−O2(S) bonds. The distribution of bond lengthsis different for the Mn2 site; there are two Mn2−O(S) bondsand four Mn2−O(L) bonds, consistent with a negative Jahn−Teller distortion. As was the case for the Mn1 configuration,the Mn2−O1 bonds are longer than the Mn2−O2 bonds.Comparing the bond distributions of the two Mn sites, it

Figure 5. (a) Local bonding environment around MnO6 site. (b)Coordination of two Mn sites, Mn1 and Mn2, in the lowest energystructure of Na2MnZr(PO4)3. Mn−O bond lengths are shown in Åbetween the Mn sites and the two distinct O sites in the structure, O1and O2. Jahn−Teller lengthened bonds are highlighted with bluearrows.

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appears that the orientation of the local Jahn−Teller distortionis subtly related to the local Na ordering, in which there is apreference to maximize the number of Jahn−Teller lengthenedbonds that share common oxygens with Na(2) sites. Althoughthe MnO6 sites are distorted on the local scale, the lack of long-range ordering of the Na(2) sites suggests that a cooperativeJahn−Teller distortion of the Mn3+ centers as observed inother materials will be inhibited, which is beneficial for thelong-term cycling stability.

■ CONCLUSIONSIn conclusion, the NASICON-structured Na3MnZr(PO4)3with three-dimensional channels for reversible extraction/insertion of Na+ ions is able to be cycled through a stable Mn3+

composition Na2MnZr(PO4)3 to give access to both the Mn4+/Mn3+ and Mn3+/Mn2+ mixed-valent two-phase states. Theoctahedral sites at the Mn3+ are distorted by different localenvironments, not by a cooperative Jahn−Teller distortion thatremoves an orbital degeneracy. Moreover, a cooperativesurface disproportionation reaction of Mn3+ is also suppressed,as there is no evidence of a Mn2+ dissolution into an organicliquid-carbonate electrolyte. On reversible extraction/insertionof two Na+ ions per formula unit, the voltage profile shows twoplateaus, one for a two-phase Mn4+/Mn3+ couple at 4.0 V andthe other for a two-phase Mn3+/Mn2+ couple at 3.5 V versusNa+/Na with a step change at the Mn3+ compositionNa2MnZr(PO4)3. This high-voltage cathode for a sodiumbattery is superior to other manganese-phosphate cathodesreported in the literature, including an earlier study that failedto extract Na+ ions reversibly from Na3MnZr(PO4)3. Thestability of the Mn3+ phase Na2MnZr(PO4)3 and the localoccupation of Na+ sites which was similar to that in NASICONstructures with a single octahedral-site cation have beenreproduced by DFT calculations. Nevertheless, still to beexplored is the role of electron tunneling from/to a surfacecoat to/from surface Na+ ions in both the Na+ extraction/insertion reactions and the penetration of electrons betweenMn ions separated by (PO4)

3− anions.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/jacs.8b11388.

Experimental details; computational methodology; TGAof the cathode material; XRD pattern and TEM image ofthe cycled cathode; structure of supercells for DFTcalculations; XPS spectra of the cathode at differentcharging state; crystallographic data of the cathodematerial; comparison with other cathode materials;magnetic moment and Bader charge calculations(PDF)

■ AUTHOR INFORMATIONCorresponding Author*jgoodenough@mail.utexas.eduORCIDHongcai Gao: 0000-0002-3671-8765Sen Xin: 0000-0002-0546-0626Leigang Xue: 0000-0002-7359-7915Graeme Henkelman: 0000-0002-0336-7153John B. Goodenough: 0000-0001-9350-3034

Author Contributions#H. Gao and I. D. Seymour contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe synthesis, analysis, and electrochemical characterization ofthe electrode material were supported by the U.S. Departmentof Energy, Office of Basic Energy Sciences (Grant number DE-SC0005397). J.B.G. and G.H. also acknowledge support fromthe Robert A. Welch Foundation (Grants F-1066 and F-1841).The calculations were done at the National Energy ResearchScientific Computing Center and the Texas AdvancedComputing Center.

■ REFERENCES(1) Goodenough, J. B. Evolution of strategies for modernrechargeable batteries. Acc. Chem. Res. 2013, 46, 1053−1061.(2) Larcher, D.; Tarascon, J. M. Towards greener and moresustainable batteries for electrical energy storage. Nat. Chem. 2015, 7,19−29.(3) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Researchdevelopment on sodium-ion batteries. Chem. Rev. 2014, 114, 11636−11682.(4) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez, J.; Rojo, T. Na-ion batteries, recent advances and presentchallenges to become low cost energy storage systems. Energy Environ.Sci. 2012, 5, 5884−5901.(5) Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F. The emergingchemistry of sodium ion batteries for electrochemical energy storage.Angew. Chem., Int. Ed. 2015, 54, 3431−3448.(6) Goodenough, J. B. Energy storage materials: A perspective.Energy Storage Mater. 2015, 1, 158−161.(7) Han, M. H.; Gonzalo, E.; Singh, G.; Rojo, T. A comprehensivereview of sodium layered oxides: powerful cathodes for Na-ionbatteries. Energy Environ. Sci. 2015, 8, 81−102.(8) Fang, C.; Huang, Y. H.; Zhang, W. X.; Han, J. T.; Deng, Z.; Cao,Y. L.; Yang, H. X. Routes to high energy cathodes of sodium-ionbatteries. Adv. Energy Mater. 2016, 6, 1501727.(9) Wang, P. F.; You, Y.; Yin, Y. X.; Guo, Y. G. Layered oxidecathodes for sodium-ion batteries: Phase transition, air stability, andperformance. Adv. Energy Mater. 2018, 8, 1701912.(10) Wang, L.; Lu, Y. H.; Liu, J.; Xu, M. W.; Cheng, J. G.; Zhang, D.W.; Goodenough, J. B. A superior low-cost cathode for a Na-ionbattery. Angew. Chem., Int. Ed. 2013, 52, 1964−1967.(11) Lee, H. W.; Wang, R. Y.; Pasta, M.; Lee, S. W.; Liu, N.; Cui, Y.Manganese hexacyanomanganate open framework as a high-capacitypositive electrode material for sodium-ion batteries. Nat. Commun.2014, 5, 5280.(12) You, Y.; Wu, X. L.; Yin, Y. X.; Guo, Y. G. High-quality Prussianblue crystals as superior cathode materials for room-temperaturesodium-ion batteries. Energy Environ. Sci. 2014, 7, 1643−1647.(13) Song, J.; Wang, L.; Lu, Y. H.; Liu, J.; Guo, B. K.; Xiao, P. H.;Lee, J. J.; Yang, X. Q.; Henkelman, G.; Goodenough, J. B. Removal ofinterstitial H2O in hexacyanometallates for a superior cathode of asodium-ion battery. J. Am. Chem. Soc. 2015, 137, 2658−2664.(14) Liu, Y.; Qiao, Y.; Zhang, W. X.; Li, Z.; Ji, X.; Miao, L.; Yuan, L.X.; Hu, X. L.; Huang, Y. H. Sodium storage in Na-rich NaxFeFe-(CN)6 nanocubes. Nano Energy 2015, 12, 386−393.(15) Singh, P.; Shiva, K.; Celio, H.; Goodenough, J. B. Eldfellite,NaFe(SO4)2: An intercalation cathode host for low-cost Na-ionbatteries. Energy Environ. Sci. 2015, 8, 3000−3005.(16) Kim, H.; Park, I.; Seo, D. H.; Lee, S.; Kim, S. W.; Kwon, W. J.;Park, Y. U.; Kim, C. S.; Jeon, S.; Kang, K. New iron-based mixed-polyanion cathodes for lithium and sodium rechargeable batteries:Combined first principles calculations and experimental study. J. Am.Chem. Soc. 2012, 134, 10369−10372.

Journal of the American Chemical Society Article

DOI: 10.1021/jacs.8b11388J. Am. Chem. Soc. 2018, 140, 18192−18199

18197

(17) Lim, S. Y.; Kim, H.; Chung, J.; Lee, J. H.; Kim, B. G.; Choi, J. J.;Chung, K. Y.; Cho, W.; Kim, S. J.; Goddard, W. A.; Jung, Y.; Choi, J.W. Role of intermediate phase for stable cycling of Na7V4(P2O7)-4PO4 in sodium ion battery. Proc. Natl. Acad. Sci. U. S. A. 2014, 111,599−604.(18) Qi, Y. R.; Mu, L. Q.; Zhao, J. M.; Hu, Y. S.; Liu, H. Z.; Dai, S.Superior Na-storage performance of low-temperature-synthesizedNa3(VO1-xPO4)2F1 + 2x (0 ≤ x ≤ 1) nanoparticles for Na-ionbatteries. Angew. Chem., Int. Ed. 2015, 54, 9911−9916.(19) Kawai, K.; Zhao, W. W.; Nishimura, S. I.; Yamada, A. High-voltage Cr4+/Cr3+ redox couple in polyanion compounds. ACS Appl.Energy Mater. 2018, 1, 928−931.(20) Masquelier, C.; Croguennec, L. Polyanionic (phosphates,silicates, sulfates) frameworks as electrode materials for rechargeableLi (or Na) batteries. Chem. Rev. 2013, 113, 6552−6591.(21) Barpanda, P.; Oyama, G.; Nishimura, S.; Chung, S. C.; Yamada,A. A 3.8-V earth-abundant sodium battery electrode. Nat. Commun.2014, 5, 4358.(22) Ong, S. P.; Chevrier, V. L.; Hautier, G.; Jain, A.; Moore, C.;Kim, S.; Ma, X. H.; Ceder, G. Voltage, stability and diffusion barrierdifferences between sodium-ion and lithium-ion intercalationmaterials. Energy Environ. Sci. 2011, 4, 3680−3688.(23) Islam, M. S.; Fisher, C. A. J. Lithium and sodium batterycathode materials: computational insights into voltage, diffusion andnanostructural properties. Chem. Soc. Rev. 2014, 43, 185−204.(24) Aravindan, V.; Gnanaraj, J.; Lee, Y. S.; Madhavi, S. LiMnPO4 -A next generation cathode material for lithium-ion batteries. J. Mater.Chem. A 2013, 1, 3518−3539.(25) Zhou, H.; Upreti, S.; Chernova, N. A.; Hautier, G.; Ceder, G.;Whittingham, M. S. Iron and manganese pyrophosphates as cathodesfor lithium-ion batteries. Chem. Mater. 2011, 23, 293−300.(26) Chen, H. L.; Hao, Q.; Zivkovic, O.; Hautier, G.; Du, L. S.;Tang, Y. Z.; Hu, Y. Y.; Ma, X. H.; Grey, C. P.; Ceder, G. Sidorenkite(Na3MnPO4CO3): A New Intercalation Cathode Material for Na-Ion Batteries. Chem. Mater. 2013, 25, 2777−2786.(27) Nisar, U.; Shakoor, R. A.; Essehli, R.; Amin, R.; Orayech, B.;Ahmad, Z.; Kumar, P. R.; Kahraman, R.; Al-Qaradawi, S.; Soliman, A.Sodium intercalation/de-intercalation mechanism in Na4MnV(PO4)-3 cathode materials. Electrochim. Acta 2018, 292, 98−106.(28) Gao, H. C.; Li, Y. T.; Park, K.; Goodenough, J. B. Sodiumextraction from NASICON-structured Na3MnTi(PO4)3 throughMn(III)/Mn(II) and Mn(IV)/Mn(III) redox couples. Chem. Mater.2016, 28, 6553−6559.(29) Wang, H. B.; Chen, C.; Qian, C.; Liang, C. D.; Lin, Z.Symmetric sodium-ion batteries based on the phosphate material ofNASICON-structured Na3Co0.5Mn0.5Ti(PO4)3. RSC Adv. 2017, 7,33273−33277.(30) Feltz, A.; Barth, S. Preparation and conductivity behaviour ofNa3MZr(PO4)3, (M: Mn, Mg, Zn). Solid State Ionics 1983, 9−10,817−821.(31) Novikova, S. A.; Larkovich, R. V.; Chekannikov, A. A.; Kulova,T. L.; Skundin, A. M.; Yaroslavtsev, A. B. Electrical conductivity andelectrochemical characteristics of Na3V2(PO4)3-based NASICON-type materials. Inorg. Mater. 2018, 54, 794−804.(32) Nogai, A. S.; Stefanovich, S. Y.; Bush, A. A.; Uskenbaev, D. E.;Nogai, A. A. Dipole ordering and ionic conductivity in nasicon-typeNa3Cr2(PO4)3 structures. Phys. Solid State 2018, 60, 23−30.(33) Pan, H. L.; Hu, Y. S.; Chen, L. Q. Room-temperature stationarysodium-ion batteries for large-scale electric energy storage. EnergyEnviron. Sci. 2013, 6, 2338−2360.(34) Yuan, L. X.; Wang, Z. H.; Zhang, W. X.; Hu, X. L.; Chen, J. T.;Huang, Y. H.; Goodenough, J. B. Development and challenges ofLiFePO4 cathode material for lithium-ion batteries. Energy Environ.Sci. 2011, 4, 269−284.(35) Chen, S. Q.; Wu, C.; Shen, L. F.; Zhu, C. B.; Huang, Y. Y.; Xi,K.; Maier, J.; Yu, Y. Challenges and perspectives for NASICON-typeelectrode materials for advanced sodium-ion batteries. Adv. Mater.2017, 29, 1700431.

(36) Wang, J. J.; Sun, X. L. Understanding and recent developmentof carbon coating on LiFePO4 cathode materials for lithium-ionbatteries. Energy Environ. Sci. 2012, 5, 5163−5185.(37) Cushing, B. L.; Goodenough, J. B. Li2NaV2(PO4)3: A 3.7 Vlithium-insertion cathode with the rhombohedral NASICONstructure. J. Solid State Chem. 2001, 162, 176−181.(38) Rangan, K. K.; Gopalakrishnan, J. AM(V)M(III)(PO4)3: Newmixed-metal phosphates having NASICON and related structures.Inorg. Chem. 1995, 34, 1969−1972.(39) Patoux, S.; Rousse, G.; Leriche, J. B.; Masquelier, C. Structuraland electrochemical studies of rhombohedral Na2TiM(PO4)3 andLi1.6Na0.4TiM(PO4)3 (M = Fe, Cr) phosphates. Chem. Mater. 2003,15, 2084−2093.(40) Jian, Z. L.; Zhao, L.; Pan, H. L.; Hu, Y. S.; Li, H.; Chen, W.;Chen, L. Q. Carbon coated Na3V2(PO4)3 as novel electrode materialfor sodium ion batteries. Electrochem. Commun. 2012, 14, 86−89.(41) Boilot, J. P.; Collin, G.; Colomban, P. Crystal structure of thetrue NASICON: Na3Zr2Si2PO12. Mater. Res. Bull. 1987, 22, 669−676.(42) Fang, Y. J.; Xiao, L. F.; Ai, X. P.; Cao, Y. L.; Yang, H. X.Hierarchical carbon framework wrapped Na3V2(PO4)3 as a superiorhigh-rate and extended lifespan cathode for sodium-ion batteries. Adv.Mater. 2015, 27, 5895−5900.(43) Song, W. X.; Cao, X. Y.; Wu, Z. P.; Chen, J.; Huangfu, K.;Wang, X. W.; Huang, Y. L.; Ji, X. B. A study into the extracted ionnumber for NASICON structured Na3V2(PO4)3 in sodium-ionbatteries. Phys. Chem. Chem. Phys. 2014, 16, 17681−17687.(44) Rui, X. H.; Sun, W. P.; Wu, C.; Yu, Y.; Yan, Q. Y. An advancedsodium-ion battery composed of carbon coated Na3V2(PO4)3 in aporous graphene network. Adv. Mater. 2015, 27, 6670−6676.(45) Jiang, Y.; Shi, J. A.; Wang, M.; Zeng, L. C.; Gu, L.; Yu, Y.Highly reversible and ultrafast sodium storage in NaTi2(PO4)3nanoparticles embedded in nanocarbon networks. ACS Appl. Mater.Interfaces 2016, 8, 689−695.(46) Kim, H.; Yoon, G.; Park, I.; Hong, J.; Park, K. Y.; Kim, J.; Lee,K. S.; Sung, N. E.; Lee, S.; Kang, K. Highly stable iron- andmanganese-based cathodes for long-lasting sodium rechargeablebatteries. Chem. Mater. 2016, 28, 7241−7249.(47) Barpanda, P.; Ye, T.; Avdeev, M.; Chung, S. C.; Yamada, A. Anew polymorph of Na2MnP2O7 as a 3.6 V cathode material forsodium-ion batteries. J. Mater. Chem. A 2013, 1, 4194−4197.(48) Park, C. S.; Kim, H.; Shakoor, R. A.; Yang, E.; Lim, S. Y.;Kahraman, R.; Jung, Y.; Choi, J. W. Anomalous manganese activationof a pyrophosphate cathode in sodium ion batteries: A combinedexperimental and theoretical study. J. Am. Chem. Soc. 2013, 135,2787−2792.(49) Gyenes, B.; Stevens, D. A.; Chevrier, V. L.; Dahn, J. R.Understanding anomalous behavior in coulombic efficiency measure-ments on Li-ion batteries. J. Electrochem. Soc. 2015, 162, A278−A283.(50) Chen, L.; Dilena, E.; Paolella, A.; Bertoni, G.; Ansaldo, A.;Colombo, M.; Marras, S.; Scrosati, B.; Manna, L.; Monaco, S.Relevance of LiPF6 as etching agent of LiMnPO4 colloidalnanocrystals for high rate performing Li-ion battery cathodes. ACSAppl. Mater. Interfaces 2016, 8, 4069−4075.(51) Choi, D. W.; Wang, D. H.; Bae, I. T.; Xiao, J.; Nie, Z. M.;Wang, W.; Viswanathan, V. V.; Lee, Y. J.; Zhang, J. G.; Graff, G. L.;Yang, Z. G.; Liu, J. LiMnPO4 nanoplate grown via solid-state reactionin molten hydrocarbon for Li-ion battery cathode. Nano Lett. 2010,10, 2799−2805.(52) Kim, H.; Yoon, G.; Park, I.; Park, K. Y.; Lee, B.; Kim, J.; Park,Y. U.; Jung, S. K.; Lim, H. D.; Ahn, D.; Lee, S.; Kang, K. AnomalousJahn-Teller behavior in a manganese-based mixed-phosphate cathodefor sodium ion batteries. Energy Environ. Sci. 2015, 8, 3325−3335.(53) Kim, S. W.; Seo, D. H.; Kim, H.; Park, K. Y.; Kang, K. Acomparative study on Na2MnPO4F and Li2MnPO4F for recharge-able battery cathodes. Phys. Chem. Chem. Phys. 2012, 14, 3299−3303.(54) Aydinol, M. K.; Kohan, A. F.; Ceder, G.; Cho, K.;Joannopoulos, J. Ab initio study of lithium intercalation in metal

Journal of the American Chemical Society Article

DOI: 10.1021/jacs.8b11388J. Am. Chem. Soc. 2018, 140, 18192−18199

18198

oxides and metal dichalcogenides. Phys. Rev. B: Condens. Matter Mater.Phys. 1997, 56, 1354−1365.(55) Jain, A.; Ong, S. P.; Hautier, G.; Chen, W.; Richards, W. D.;Dacek, S.; Cholia, S.; Gunter, D.; Skinner, D.; Ceder, G.; Persson, K.A. Commentary: The Materials Project: A materials genome approachto accelerating materials innovation. APL Mater. 2013, 1, 011002.(56) Chotard, J. N.; Rousse, G.; David, R.; Mentre, O.; Courty, M.;Masquelier, C. Discovery of a sodium-ordered form of Na3V2(PO4)3below ambient temperature. Chem. Mater. 2015, 27, 5982−5987.(57) Delmas, C.; Nadiri, A.; Soubeyroux, J. L. The nasicon-typetitanium phosphates ATi2(PO4)3 (A = Li, Na) as electrode materials.Solid State Ionics 1988, 28, 419−423.(58) d’Yvoire, F.; Pintard-Screpel, M.; Bretey, E.; de la Rochere, M.Phase transitions and ionic conduction in 3D skeleton phosphatesA3M2(PO4)3: A = Li, Na, Ag, K; M = Cr, Fe. Solid State Ionics 1983,9−10, 851−857.(59) Cherkaoui, F.; Viala, J. C.; Delmas, C.; Hagenmuller, P. Crystalchemistry and ionic conductivity of a new Nasicon-related solidsolution Na1+xZr2-x/2Mgx/2(PO4)3. Solid State Ionics 1986, 21,333−337.(60) Shannon, R. D.; Prewitt, C. T. Effective ionic radii in oxides andfluorides. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem.1969, 25, 925−946.(61) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B.Phospho-olivines as positive-electrode materials for rechargeablelithium batteries. J. Electrochem. Soc. 1997, 144, 1188−1194.(62) Jian, Z. L.; Yuan, C. C.; Han, W. Z.; Lu, X.; Gu, L.; Xi, X. K.;Hu, Y. S.; Li, H.; Chen, W.; Chen, D. F.; Ikuhara, Y.; Chen, L. Q.Atomic structure and kinetics of NASICON NaxV2(PO4)3 cathodefor sodium-ion batteries. Adv. Funct. Mater. 2014, 24, 4265−4272.(63) Cerrato, J. M.; Hochella, M. F.; Knocke, W. R.; Dietrich, A. M.;Cromer, T. F. Use of XPS to identify the oxidation state of mn in solidsurfaces of filtration media oxide samples from drinking watertreatment plants. Environ. Sci. Technol. 2010, 44, 5881−5886.(64) Tang, W.; Sanville, E.; Henkelman, G. A grid-based Baderanalysis algorithm without lattice bias. J. Phys.: Condens. Matter 2009,21, 084204.(65) Goodenough, J. B. Jahn-Teller phenomena in solids. Annu. Rev.Mater. Sci. 1998, 28, 1−27.(66) Yamada, A.; Tanaka, M.; Tanaka, K.; Sekai, K. Jahn-Tellerinstability in spinel Li-Mn-O. J. Power Sources 1999, 81, 73−78.(67) Piper, L. F. J.; Quackenbush, N. F.; Sallis, S.; Scanlon, D. O.;Watson, G. W.; Nam, K. W.; Yang, X. Q.; Smith, K. E.; Omenya, F.;Chernova, N. A.; Whittingham, M. S. Elucidating the nature ofpseudo Jahn-Teller distortions in LixMnPO4: Combining densityfunctional theory with soft and hard X-ray spectroscopy. J. Phys.Chem. C 2013, 117, 10383−10396.

Journal of the American Chemical Society Article

DOI: 10.1021/jacs.8b11388J. Am. Chem. Soc. 2018, 140, 18192−18199

18199